1. Field of the Invention
The present invention relates to an ultrasonic wave motors in which a moving member is driven by a travelling vibration wave generated in a resilient member by a piezoelectric member.
2. Related Background Art
As disclosed in the U.S. Pat. No. 4,510,411, ultrasonic wave motor utilizing a travelling vibration wave generates a travelling vibration wave in a resilient member by applying an AC voltage to a piezoelectric member attached to the resilient member, thereby causing bending vibration in the piezoelectric member, and drives a rotor by maintaining the rotor in pressure contact with the resilient member.
The driving speed of such ultrasonic wave motor has been conventionally controlled by the frequency of the applied AC voltage in such a manner that the output voltage from a monitor electrode provided on the piezoelectric member remains at a predetermined level, or in such a manner that the phase difference between the output of the monitor electrode and the applied AC voltage becomes constant.
However, such frequency control to obtain constant monitor voltage or phase difference is sometimes still unable to provide desired speed, due to change in load, fluctuation in the ambient conditions or fluctuation in the pressure mentioned above.
In the field of DC motors, there is already well known control by a phase locked loop (PLL) for obtaining a desired speed in a stable manner. However, since the driving mechanism of the ultrasonic wave motor is different from that of the DC motor, the PLL circuit for a DC motor cannot be immediately applied to the ultrasonic wave motor.